An underground drilling process may be monitored and information relating to the drilling process can be transmitted to a receiver that is located above the ground. One transmission technique known as electromagnetic telemetry (EM) uses low frequency (few hertz) transmission of information through a geological formation that is being drilled. U.S. Pat. No. 7,252,160 of Dopf et al illustrates a prior art drilling system that has an EM telemetry module, and especially a band gap. The band gap electrically insulates two elements of an antenna. The antenna should be large in order to be effective in the low frequency range.
The information that is to be transmitted above the surface is usually encoded in a time-base pulse scheme or by modulation of a carrier wave.
Some prior art EM tools have only been optimized to operate in open hole conditions where geological formation impedance typically exceeds one ohm.
Recent EM tools should be expected to operate in the low impedance geological formations that exhibit an impedance of much less than one ohm.
Some prior art EM tools have used transformer and inductor-based converters which do not cope well with these low impedance geological formations, producing little output at low efficiency with such loads.
New types of EM tools are needed to produce higher output in these low impedance conditions that work at higher efficiency which allow the EM tools to operate longer downhole.